Since polyester has excellent mechanical strength, chemical stability and transparency, as well as being inexpensive, it is one of the most commonly used synthetic resins throughout the world in various applications, including fibers, films, sheets and containers. Of all kinds of polyester, polyethylene terephthalate is particularly advantageously used because of its excellence in general versatility and practical applicability.
Generally speaking, polyethylene terephthalate is manufactured from terephthalic acid, or an ester-forming derivative thereof, and ethylene glycol, and such raw materials are normally obtained from fossil resources. Although oil, a fossil resource, is an important raw material in the chemical industry, it is a cause of global warming and other environmental problems as it generates large quantities of carbon dioxide during manufacturing and incineration disposal, not to mention concerns over future depletion. Such being the case, much attention has been focused on the use of reclaimed materials and materials with a low environmental load.
Biomass resources are produced by plants from water and carbon dioxide through photosynthesis, and take the forms of starch, carbohydrate, cellulose, lignin, and the like. Since biomass resources take in carbon dioxide as one of their input materials during their formation processes, any material that uses a biomass resource does not produce any net carbon dioxide emissions in its life cycle, even if decomposed into carbon dioxide and water during post-use incineration disposal. As this carbon dioxide may, under certain circumstances, be recycled by plants, biomass resources can be regarded as renewable resources. Using such biomass resources as an alternative to oil resources helps preserve fossil resources and reduce carbon dioxide emissions.
Against this background, ways to synthesize polyester, a very high-demand polymer, from renewable biomass resources are being studied. Examples include a report on polyethylene terephthalate (PET) synthesized from biomass-derived ethylene glycol (Chinese Patent Publication No. 101046007). However, since biomass-derived ethylene glycol is low in purity, any polymer obtained from it exhibits a problem of thermostability in the form of a low melting point.
As a method to overcome this problem, an adsorption treatment designed to remove impurities from biomass-derived ethylene glycol using activated carbon has been disclosed (Japanese Unexamined Patent Publication (Kokai) No. 2009-209145). That method has made it possible to obtain polymers with melting points comparable to those synthesized from fossil resource-based glycols.
After chipping, polyester is usually melted again and molded, and is subjected to a thermal history approaching 300° C. in the process. Compared to fossil resource-based polymers, polymers synthesized from biomass-derived glycol have poor thermostability. In this regard, we observed a problem in that the above process promotes decomposition reaction in such polymers and causes yellowing and a reduction in viscosity, i.e., a reduction in molecular weight, leading to several undesirable phenomena, including an increased soiling of the die of the molding machine and generation of foreign matter.
It could therefore be helpful to provide a polyester that has excellent thermostability during melt molding, namely, to provide a polyester with only a small reduction in intrinsic viscosity during melt molding, as well as a manufacturing method therefor.